Pressure sensitive adhesives are well known in the art. An "adhesive," as defined by C. G. Hawley in "The Condensed Chemical Dictionary," 9th Ed., Van Nostrand Reinhold, New York, N.Y., 1977, is "any substance, inorganic or organic, natural or synthetic, that is capable of bonding other substances together by surface attachment." A pressure sensitive adhesive (psa) is generally a component of a pressure sensitive tape which in its most simple configuration is composed of an adhesive and a backing. The overall construction is tacky and adherent at the use temperature (typically room temperature) and adheres to a variety of substrates using only moderate (typically fingertip) pressure to form the bond. In this fashion, pressure sensitive tapes constitute a complete, self-contained bonding system.
A good psa, according to D. W. Aubrey in "Developments in Adhesives," W. C. Wake, Ed., Applied Science Publishers, London, England, Vol. I, chapter 5, must fulfill three main technical requirements:
1. The adhesive must exhibit adequate resistance to shear under the influence of an applied load; this relates to the adhesives's cohesion. PA1 2. The adhesive must conform quickly to the surface to which it is applied in order to rapidly form a bond; this relates to tack. PA1 3. The adhesive must display adequate resistance to separation by peeling once the bond is formed; this requirement relates to adhesion. PA1 (1) at least one condensation-based polyamide, and PA1 (2) at least one pressure sensitive acrylic polymer.
These three requirements are assessed generally by means of tests which are designed to measure cohesion (shear holding power), tack, and adhesion (peel strength). These measurements taken together constitute the "balance of properties" often used to characterize psa's.
Homopolymers of long chain alkyl acrylates are quite tacky but lack the cohesive strength necessary to function as psa's having well-balanced properties. This deficiency may be overcome by copolymerization with polar monomers capable of interpolymeric dipole-dipole or hydrogen bonding interactions as disclosed in U.S. Pat. No. 2,884,126. Whereas this approach is adequate for some applications, further increases in cohesive strength is needed in the more demanding applications, such as those requiring load-bearing ability. It has been the common practice in the art to obtain this increased cohesion by crosslinking of the adhesive. Numerous methods have been described for crosslinking, and all of them suffer certain disadvantages (see Aubrey, op. cit., pp. 146-148). Therefore, it would be advantageous to have a simple method for improving cohesive strength without the need for crosslinking reactions with their attendant problems.
U.S. Pat. No. 4,370,380 teaches that certain pressure sensitive acrylic copolymers, which, because of their low molecular weights, do not have sufficient cohesive strengths to be satisfactory psa's by themselves, may be improved in cohesive strength by blending with 1-30% by weight of vinylactam polymers or copolymers, such as polyvinylpyrrolidone.
Condensation-based polyamides, e.g. nylon, are well known in the art and in general are film- and fiber-forming thermoplastic materials. Certain polyamides, e.g., those based on polymerized fatty acids, also known as dimer acids, are useful as hot-melt adhesives (see D. E. Peerman in Encyclopedia of Polymer Science and Technology, Wiley, N.Y., 1969, Vol. 10, pp. 597-615). These polyamides in general are not useful as psa's, because they are not tacky at room temperature. Psa's may be prepared however, by blending polyamides with materials such as colophonium resins, N-substituted toluenesulfonamides, or phthalate esters, as taught in U.S. Pat. No. 3,792,002.
Blends of polyamides and acid-containing olefin polymers are also known in the art and are useful as hot-melt adhesives or molding resins. U.S. Pat. Nos. 3,845,163 and 4,018,733 describe blends of polyamides with ethylene/acrylic acid or ethylene/methacrylic acid copolymers. Blending of minor amounts of normally non-tacky acrylic copolymers with condensation-based polymers to improve impact resistance of these thermoplastic polymers is taught in U.S. Pat. Nos. 4,167,505 and 4,395,518.